1. Field of the Invention
The field of the invention is that of a power line switching circuit for a communication interface line between a host computer and peripheral system. The present invention is specifically advantageous for interfaces using the American National Standard for Information Systems Small Computer System Interface Specification or (ASCII/SCSI) X3T9.2/82-2.
2. Description of Related Art
Many peripheral systems are designed to interface with host computers, specifically the Apple Macintosh.RTM. computer "(Macintosh.RTM."). These peripheral systems provide extended capability for the Macintosh.RTM. computer by providing expanded memory, magnetic or optical disk data storage, printing means, image scanning ability, and backup disk data storage (tape drive). Referring now to prior art FIG. 1(A), generally a peripheral system 220 includes a peripheral device 210 (in this case a hard drive), an internal power supply 200 for the peripheral device 210 and various connecting and interfacing means 212 that enable the peripheral system 220 to communicate with other systems. All of these components are within one outer covering and are collectively called a "peripheral" or a "peripheral system" for a host computer.
Peripheral systems connect directly to the host computer and communicate with the host by way of a communication interface or protocol which will be expanded upon further below. Designers of peripheral systems create their systems to operate with the particular communication interface accepted by the host computer. Therefore, host computer users can universally connect to the host computer any manufacturer's peripheral system that uses the host's same communication interface. This interface method greatly expands the scope of peripheral compatibility with the host computer. The communication interface may be comprised of a wire cable, interface hardware, and special processing software. All three elements are uniquely designed to operate under a specific communication protocol. The wire cable described connects the peripheral system to the host computer by a physical means and is made up of many independent wires (also called lines or pins). Together the interface hardware and software provide the mechanism for sending and receiving binary data which is electrically transmitted over the wire cable between the host computer and the peripheral system.
The communication protocol advantageously used with the present invention is the SCSI (pronounced "SKUH-zee") standard which is used in the Apple Macintosh.RTM. line of computers (Macintosh.RTM. Plus, Macintosh.RTM. SE, and Macintosh.RTM. II family) and has also been widely adapted by many other brands of computers via plug-in adapters to communicate with the host systems. SCSI stands for Small Computer Systems Interface and is a high speed, parallel communication interface defined by the American National Standards Institute. The SCSI interface is designed to connect a variety of tape drives, printers, disk drives, scanners, and other peripherals with the Macintosh.RTM. computer. The SCSI wire cable is made up of a plurality of different insulated, parallel lines (or pins) all of which are connected on both ends with one of a variety of interface connectors. The connector adapted for the Macintosh.RTM. computer, illustrated in FIG. 1(B), is made tap of 25 pins. On the peripheral system side of the interface, the connector adapted for the peripheral system has 50 pins (25 of these are grounded and not used). One of these SCSI pins is designated as a " terminator power pin" (or "power pin") and preferably carries a 4.5 to 5.5 volt DC power level. Refer to FIG. 2(B). The function of the terminator power pin is to provide power for the termination resistor network 80 typically located at the end of the SCSI bus in a chain of peripherals. The terminator resister network 80 conditions certain signal lines on the SCSI bus. The terminator resistor network 80 can be built into a peripheral system, or can be applied to the peripheral as an external module that plugs into the SCSI bus at the end of a peripheral chain, see arrangement in FIG. 2(B). The terminator power pin on the Macintosh.RTM. computer side of the interface is illustrated on FIG. 1(B) as pin number 25. As shown by the present invention in FIG. 2(A), the SCSI connector is adapted to fit into the Macintosh.RTM. computer SCSI port 107 located on back of the computer 104. The other connector is placed into the peripheral system's communication port 82 located on the back of the peripheral system 102.
It has been discovered that one problem with current peripheral system design is that the possibility of short circuiting the peripheral devices through the SCSI interface has not been considered fully. Refer to the prior art FIG. 1(A). To date, within the peripheral system 220, peripheral device 210 is connected to peripheral power source 200. The peripheral device 210 is also directly connected to the terminator power pin of the SCSI communication line 212 via a diode 214 which prevents current flow back into the peripheral device 210. In order to power the SCSI interface terminator power pin, power from the peripheral power source 200 is channeled through the peripheral device 210 then through the current regulating diode 214. From the diode 214, the power flows to the terminator power pin of the SCSI interface cable 212. Thus, it has been discovered that short circuits may occur when either (i) the cable is inserted incorrectly or (ii) before it is connected to the prior art peripheral system 220, the terminator power pin already carries power from some alternate source, but at a voltage lower than that supplied by peripheral system 220, causing excessive current to flow through diode 214. The resulting short circuit may create a damaging current surge through (and out of) the peripheral device 210 that could render the peripheral system 220 inoperative.
As one aspect of the present invention, it is realized that the short circuiting problem of the SCSI communication interface exists at least because on the Macintosh.RTM. computer the power state of the terminator power pin is unknown. Further, it is now realized that many short circuits may be avoided with knowledge of the power state of this terminator power pin. The Apple Macintosh.RTM. line of computers consists of several different models (Macintosh.RTM. Plus, Macintosh.RTM. SE, and Macintosh.RTM. II family), on these various models some of their SCSI interfaces carry power on file terminator power pin and some models do not, leaving terminator power pin disconnected. For example, one model of Macintosh.RTM. computer that does not supply power on the terminator power pin, leaving it unconnected, is the Macintosh.RTM. Plus. The nonuniform power state of terminator power pin causes a major problem for peripheral designers because the power state of the terminator power pin on the SCSI communication interface for the Macintosh.RTM. computer is unknown or ambiguous. To deal with this problem, peripheral manufactures connect the SCSI terminator power pin to the peripheral's internal power source. However, since several families of Macintosh.RTM. computers also independently place power on the terminator power pin, these two power sources (connected in parallel) will compete to level their voltages.
The short circuit problem becomes more acute when several peripherals (via the SCSI communication interface) are connected to the Macintosh.RTM. computer in "daisy chain" fashion as shown in FIG. 2(B). The host computer 104 is connected to peripheral "one" 102 via SCSI cable 110, then from peripheral "one" 102 to peripheral "two" 109 via SCSI cable 112. Both peripherals 102,109 use the same SCSI interface line to communicate with the host computer 104. Since all SCSI lines are configured in parallel in the daisy chain scheme, power from one peripheral power supply (located in peripherals 102 and 109) will be applied to the SCSI terminator power pin and across all components in the daisy chain, including the Macintosh.RTM. computer 104. If several peripheral systems are attached in parallel then the power supplies of all peripherals will also be connected in parallel via the terminator power pin of the SCSI cable. Again, it is realized by the present invention that a problem exists because peripheral power supplies connected in parallel will increase the probability of: short circuit failures; excessive forward bias current through the diodes; and/or current surges as each power supply operates to force the other parallel voltage to a constant and equalized level.
In reference to the above, peripheral manufactures cannot anticipate whether or not there will be power on the terminator power pin of the SCSI connector because: (i) not all host computers (such as the Macintosh.RTM. computer) supply power on the terminator power pin; and (ii) the daisy chain parallel configuration of multiple peripheral systems allows any peripheral system to supply power across terminator power pin of the entire SCSI interface. Furthermore, the design of present peripheral systems provides no mechanism for dealing with the ambiguous power levels associated with the terminator power pin of the SCSI interface.
Refer to FIG. 1(A). Additionally, the protective features of prior art peripheral systems 220 are inadequate to protect the peripheral devices 210 from short circuits related to the SCSI terminator power pin. This type of short circuit causes current to surge out of the peripheral device 210 thus damaging its internal components. In the prior art, the current regulating diode 214 only protects the peripheral device 210 from current flow into the peripheral device 210 and not current flow out of the peripheral device 210. However, the present invention discloses methods and means preventing hazardous current flow out of a peripheral system onto a SCSI interface line which may permanently damage the peripheral system.
It is therefore an object of the present invention to develop a new circuit that can sense the power level on a terminator power pin of a SCSI line using a monitor such as a light emitting diode (LED). Depending on this power level, the present invention provides a switch to either connect an internal peripheral power source to or disconnect it from the SCSI communication line. Once the peripheral power source is disconnected from the SCSI line, no shorting condition can arise as a result of current from the SCSI terminator power pin. The advantage to this arrangement is that if power is not supplied on the SCSI line from the Macintosh.RTM. computer, or from any other peripheral device connected on the SCSI bus, then the peripheral LED will not light and the switch can be used to connect the peripheral's internal power source to the terminator power pin of the SCSI line and the SCSI interface will operate normally. Alternatively, if power already exists on the SCSI terminator power pin then the peripheral LED will illuminate and no power needs to be supplied from the peripheral system. The switch is then open to disconnect the peripheral's internal power source from the SCSI line. In this configuration no shorting can occur in the peripheral system as a result of the SCSI terminator power pin since the power lines have been disconnected.
Another object of the present invention is to provide a circuit that does not connect power lines (terminator power pin) between the peripheral device and the SCSI communication interface, as practiced by the prior art. As seen in the prior art FIG. 1(A), the peripheral device 210 is coupled directly to the terminator power pin of the SCSI interface connector 212 by a power line. This power line coupling scheme is not implemented in the present invention. Instead the present invention, by decoupling the power lines between the SCSI interface and the peripheral device, increases the reliability of the entire peripheral system by lessening the chances of a peripheral device short circuit through the SCSI interface terminator power pin.
Another object of the present invention is to increase peripheral system reliability by protecting the peripheral system 102 from current surges through the communication port by utilizing a replaceable fuse coupled to the SCSI terminator power pin. The prior art, refer to FIG. 1(A), does not provide a fuse link on the power line between the SCSI port 212 and the peripheral device 210. Therefore, the present invention has additional reliability over the prior art due to the fuse application.